This invention relates to methods and apparatus for stabilizing feedback control systems, having particular application in audio-signal amplifiers.
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Commercially available electronic sound systems are known to reproduce music that lacks the musicality of live performances. More expensive audio systems reproduce slightly better music, but are still unsatisfactory. Components of electronic sound systems include power amplifiers, pre-amplifiers, and program players. They are, in essence, electronic amplification systems using closed-loop control principle or feedback-controlled amplifiers for short.
The signal distortion that reproduces bad music comes from many causes. Major causes are nonlinear transfer function of electronic circuits (such as crossover distortion), feedback instabilities and speaker imperfections. Feedback instabilities are most difficult to understand. They are transient and not easily observable on a test bench using resistive loads and steady state input signals. They do occur during operation with real speakers and transient musical signals.
Because of the difficulty in observation and understanding of instabilities, designers do not recognize the problem. Previous circuits for stabilization comprise resistor-capacitor networks that remain basically the same for years. They are experimentally designed to suppress oscillations observed on bench testing. They comprise mainly a compensation capacitor that rolls off high frequency gain. This capacitor is connected from output to feedback terminal. Power amplifiers often have a damping inductor termination at the output; followed by a series resistor-capacitor connected to ground. Capacitors arc also connected from collector to base of bipolar transistors, and resistors inserted to base and emitter. They offer little help in reducing transient instabilities.
Recently U.S. Pat. No. 5,825,250 (1998) to Tomasini, et al. disclosed a selectively switched capacitor network to replace the conventional fixed-value compensation capacitor. The purpose is to match the compensation capacitor to the closed-loop gain. When the feedback resistor changes value, the compensation capacitor network would switch to a new value to ensure sufficient stable bandwidth. This variable compensation may extend the bandwidth a little but is still the old fashion frequency compensation that will not prevent transient instabilities. Further more, amplifiers that use fixed closed-loop gain, such as audio amplifiers do not need variable compensation. The problem is deeper than that, as will be explained in the detailed description of the present invention.
Unaware of transient instabilities, designers of sound systems addressed other problems without improving sound quality:
1. Problems such as suppression of noises and electromagnetic interferences that may pollute input signals and power supplies. As sound quality is concerned, commercially available boxes offering to cleanup AC power sources are unsatisfactory.
2. Recently, problems resulted from transmission line effects are addressed in U.S. Pat. No. 4,885,555 (1989) and U.S. Pat. No. 5,222,149 (1993), both to Palmer. The apparatus disclosed by Palmer are essentially terminations using transformer effect either for impedance matching or for dissipation of ringing signals in the transmission line. The primary circuit is the signal carrying line and the secondary circuit is shorted. The apparatus are located at strategic locations along the transmission line for proper dissipation of ringing signals. Commercially available speaker cables and interconnects having complex woven patterns also produce expensive and unsatisfactory audio accessories. They are also designed to manipulate standing waves in cables considered as transmission lines.
3. Methods for improving sound quality also include selection of parts. Selection of parts often calls for exotic devices that bring about small improvement. Examples of exotic devices include special vacuum tubes, pure resistors, low-dissipation capacitors, oxygen-free copper, silver wires, gold-plated terminals. Sound quality improvement may exist between a low cost solid-state amplifier and an expensive tube amplifier. However, the small improvements cannot justify additional costs, except for a few wealthy people.
4. A very expensive speaker may sound a little better than the one costing half as much. However, the expensive speaker falls into the range of extreme diminishing return.
In conclusion, prior arts were unsatisfactory for improvement of sound quality and did not address an important cause of distortion, namely feedback instabilities.
The present invention addresses the problem of amplifier feedback instabilities uncorrected by conventional stabilization.
It is a primary object of the invention, therefore, to provide a system for improving stability of feedback-controlled amplifiers, particularly to enrich the musicality of audio-signal amplifiers. Other objects and advantages are:
To provide a cost-effective system for improving sound quality,
To provide an add-on system for speaker terminals and output jacks of program players of sound systems already in use.
Still further objects and advantages will become apparent from a consideration of the ensuing description and drawings.
In accordance with the present invention a method for stabilizing feedback-controlled amplifiers comprises inserting an apparatus having controllable impedance, typically, from output to ground, and applying an unstable signal (typically, the unstable output voltage itself) to control the impedance toward values where instabilities are reduced (or they cannot grow larger).
Typically the apparatus is a network having multiple configurations selected by switches. Each configuration has properly designed impedance. Switches can be transistor switches controlled by unstable signals toward configurations designed to minimize unwanted oscillations. The unstable signals should control the switches through the intermediary of a computer code that can provide prediction capability to act earlier, at the onset of instabilities.